Various image emphasis processing and noise reducing processing have been hitherto developed to enhance visibility of medical images. For example, Patent Document 1 discloses, as image emphasis processing, a technique of extracting a characteristic of each image portion and selecting a filter function corresponding to the characteristic to perform emphasis processing. Furthermore, Patent Document 2 discloses a technique of investigating the density gradation of points around a target point (=points in the neighborhood of the target point), calculating an average value of pixel values in the gradation maximum direction and emphasizing an image by using the average value when the target point is emphasized.
Furthermore, Patent Document 2 discloses a technique of separating an image into an edge portion and an flat portion and performing smoothing processing which is matched with the characteristic of each image portion.